1. Field of the Invention
The present invention relates generally to separation analysis of a sample, and more particularly to an analytical apparatus that uses an ion-trap mass spectrometer.
2. Description of the Related Art
A typical ion trap mass spectrometer employs a pair of opposing bowl-like end cap electrodes and toroidal ring electrodes between the end cap electrodes. Quartz rings, for example, are used between the electrodes as spacers for maintaining predetermined intervals between the electrodes. A plurality of small openings (about 3 mm in diameter) are bored in the spacers, through which a buffer gas is introduced and through which the mass spectrometer can be evacuated.
The buffer gas is an indispensable gas introduced to converge the trajectories of the ions injected into the ion trap mass spectrometer. The pressure of the buffer gas in the spectrometer is kept at approximately 10.sup.-3 -10.sup.-4 torr to optimize the efficiency of ion convergence. A pump is used to control the pressure in a high-vacuum chamber containing the mass spectrometer and the size of the openings in the spacers.
A detector and an ion focusing lens are also provided in the high-vacuum region in the spectrometer. The high-vacuum region prevents electrical discharge at the detector due to the high voltage applied to the detector.
Periodically, the ion trap mass spectrometer requires inspection and maintenance, including decontamination, which is performed by exposing the mass spectrometer to atmospheric pressure. Following the inspection and maintenance, approximately 10-12 hours are required to evacuate the mass spectrometer through the small openings in the spacers, to reach the target pressure of approximately 10.sup.-3 to 10.sup.-4 torr. The openings cannot be enlarged more than the noted diameter because the buffer gas pressure must be kept at a level at which the efficiency of convergence is optimized.
Mordehai et al, "A Novel Differentially Pumped Design for Atmospheric Pressure Ionization-ion trap Mass Spectrometry" (Rapid Communications in Mass Spectrometry, Vol. 7, 205-209 (1993)) describes an apparatus that provides the detector in a separate chamber at a succeeding stage of a differential pumping area, to effect the internal evacuation by a different pump. By this design, the evacuation time following inspection and maintenance is to be reduced, because the ionization is performed at atmospheric pressure.